1. Field of the Invention
This invention relates to a method for releasing a drill string which has become stuck in the borehole of an underground formation.
2. Description of the Prior Art
During drilling operations the drill string may become stuck and cannot be raised, lowered, or rotated. There are a number of mechanisms possible which may contribute to this problem. Namely these are () cuttings or slough build-up in the hole, (2) an undergage hole, (3) key-setting and (4) differential pressures.
This invention relates particularly to the differential pressure problem and is intended to be a novel approach to alleviating this situation.
Differential sticking may be defined as the sticking of the drill string against a permeable formation containing less pore fluid pressure than the hydrostatic pressure exerted by the drilling fluid column and usually occurs when the drill string remains motionless for a period of time. The mechanism by which this occurs involves the drill string coming into contact with the permeable zone, remaining quiescent for a period of time sufficient for mud cake to build up on each side of the point of contact, thus sealing the pipe against the borehole. The annular pressure exerted by the drilling fluid then holds the pipe against the borehole or the permeable zone.
Freeing of differentially stuck pipe is essentially a matter of reducing this pressure differential which exists across the pipe. One method used simply involves a reduction in fluid pressure by replacing the annular fluid wwith a less dense fluid allowing for less pressure differential to exist between the borehole and annulus. In some cases the borehole pressure may exceed the annular pressure which in turn allows the pipe to be blown out of the borehole.
One commonly used method to release stuck pipe is the spotting of an oil base mud in the hole opposite the stuck interval. With time, the mud cake around the stuck pipe becomes dehydrated from theoretically, hydraulic forces that are applied, the cake cracks and falls away freeing the pipe. Too, the oil mud may penetrate between the mud cake and pipe lubricating the area between the pipe and borehole resulting in less friction and quicker release. More often than not, an extensive period of time is necessary for the release to occur which results in an expensive loss of rig time.
In recent years, there has been a number of proprietary formulations developed aimed at releasing differentially stuck pipe. These also seem to require a great deal of time and the success ratio is somewhat questionable. U.S. Pat. No. 4,230,587, C. O. Walker, describes a method for releasing stuck drill pipe utilizing polyethylene glycol. U.S. Pat. No. 3,217,802 (Reddie et al) uses a petroleum base fluid with an alcohol coupling agent for the same purpose.